Voltage control at windfarms

ABSTRACT

A voltage control arrangement for a system of multiple windfarms with transmission lines. Voltage is regulated at a point of regulation on the system, such as a high voltage substation or other system bus. Regulation is achieved at the point of regulation by sensing the voltage, comparing to a reference voltage, and adjusting the reactive power output of the wind turbines and other equipment in the system. The regulation point may be shifted to another point if needed to respect voltage limits at that points of the system after attempting to shift reactive load to restore voltage within limits at the other points in the system. The reference voltage may be adjusted to minimize losses for the system of multiple windfarms and transmission lines. A loss optimizing algorithm is applied to the combined multiple windfarm and transmission line to shift reactive load among local windfarms to minimize losses and to shift reactive load among individual wind turbines within an individual windfarm.

BACKGROUND OF THE INVENTION

The invention relates generally to operation of windfarms within anelectric grid and more specifically to voltage control for a system ofmultiple windfarms.

Typically, an electric power system includes a plurality of powergeneration assets, which are spread over a geographic area. The electricpower system also includes systems that consume power (loads) that mayalso be spread over the geographic area. The electric power system alsoincludes a grid, a network of electric power lines and associatedequipment used to transmit and distribute electricity over a geographicarea. The infrastructure of the grid, may include, but is not limited todevices for interconnection, control, maintenance, and improvement ofthe electric power system operation. Typically, the electric powersystem includes a centralized control system operatively connected tothe power generation assets for controlling a power output of each ofthe power generation assets, for example, using processing logic. Thenetwork operator usually operates the centralized control system. Thepower output of the power generation assets controlled by thecentralized control system may include, but is not limited to an amountof electrical power, and a voltage for the electrical power.

Wind energy is often used to generate electrical power at power plants,often referred to as windfarms, using, for example, the rotation oflarge wind turbines to drive electrical generators. Windfarms and theirassociated windfarm controllers can control reactive power supply, andto a more limited extent active power. Larsen, in U.S. Pat. Nos.7,119,452, 7,166,928, and U.S. Pat. No. 7,224,081 (assigned to GeneralElectric Co.) describes a voltage control for wind generators includinga farm-level controller with a reactive power command and a wind turbinegenerator control system. Wind turbine generator voltage control may beprovided by regulating the voltage according to a reference set by ahigher-than-generator-level (substation or farm level) controller.Reactive power may be regulated over a longer term (e.g. few seconds)while wind turbine generator terminal voltage is regulated over ashorter term (e.g. fraction of a second) to mitigate the effect of fastgrid transients.

For economic reasons and as one of the approaches to reduce theenvironmental impacts of fossil fuel power generation, wind turbinegenerators with larger power output are being produced and windfarmswith greater numbers of wind turbine generators are being brought intooperation. The power output from the windfarms may comprise asignificantly larger part of the total power being supplied andtransmitted along the transmission grid. Often, an original windfarm maybe sited at a certain geographic location, based on desirable windconditions at that location. Later, one or more additional windfarms maybe sited at the same geographic area, based on the desirable windconditions that motivated the first windfarm. The later windfarms may bebuilt by the same operator as the first windfarm or by completelydifferent operators. The outputs from windfarms may be interconnected ina variety of points, which are ultimately tied together at a point ofcommon coupling. The point of common coupling may also be the point ofinterconnection to the electric power system grid. The point of commoncoupling may provide a location for measurement of combined outputparameters from the plurality of interconnected windfarms.Alternatively, the point of common coupling may be removed from thepoint of interconnection with grid. Increasingly, as windfarms are beinglocated in geographic areas with valuable wind characteristics, thewindfarms are remote from existing transmission lines of a grid.Increasingly, transmission lines of up to hundreds of miles need to beconstructed to tie newly-built windfarms into the existing grid.

The interconnection of the windfarms in the windfarm system may be indifferent configurations. The distances between the windfarms may vary.Further, the point of physical connection with the grid may be remotefrom any of the individual windfarms and the point of common coupling.In the case of the plurality of interconnected windfarms with individuallocal windfarm controllers, individual local power-related commands maybe provided to the individual local windfarm controllers from thecentral control system. Typically, the power-related commands providedto the local windfarm controller may direct the local windfarmcontroller to provide a specific power-related output at the point ofcommon coupling. However, the plurality of individual local windfarmcontrollers cannot control at the point of common coupling because thepower-related parameters at that point are a combination of the outputsfrom all of the individual windfarms.

Prior art windfarm systems have incorporated regulation of the voltageoutput from multiple windfarms at a location of measurement or a pointof common coupling, For example, Cardinal et al. (U.S. application Ser.No. 12/181,658 assigned to General Electric Co.) describes a masterreactive control device for regulating voltage output from multiplewindfarms at a point of common coupling or a point of interconnectionwith a grid. In other instances, the regulation of voltage output formultiple windfarms at a point of regulation distant from the location atwhich the parameters may be measured, for example at the point ofinterconnection with the grid. However, voltage regulation at a singlepoint associated with the output from the multiple windfarms may lead toviolation of voltage limits at other locations on the transmission lineor within individual windfarms. FIG. 1 illustrates a voltage profile atvarious points on a transmission line between a point of common coupling(POCC) with a windfarm and a point of interconnection (POI) with a grid.Planning criteria may typically require that rated power from thewindfarm be delivered at the POI for voltages in the range of 0.95 powerunit (PU) to 1.05 PU. However, depending upon the fraction of ratedpower and compensation schemes employed, voltage will vary along thetransmission line between the POCC and the POI with the grid. It wouldbe desirable to be able to control output of windfarms to maintainpoints on the transmission line within voltage specification duringtransmission line operation. Voltage or other limits could similarly beviolated at other points in the system of windfarms, such as atcollector bus outputs for individual windfarms.

Further, prior art has incorporated methods for distributing reactiveload and windfarm voltage optimization for reduction of collector systemlosses within individual windfarms such as in Cardinal et al. (U.S.application Ser. No. 12/039028 assigned to General Electric Co.). FIG. 2illustrates a prior art system for minimizing losses within a singlewindfarm by distribution of reactive power commands to individual windturbines utilizing a loss minimization algorithm. The windfarm collectorsystem 200 shows three wind turbine generators 201, 202, and 203,however, the number of wind turbine generators may be broadly extendedin practical application. The wind turbine generators 201, 202 and 203provide outputs P₁+jQ₁ (207), P₂+jQ₂ (208) and P₃+jQ₃ (209). Each windturbine generator 201, 202 and 203 is tied to a collector bus 205through a wind turbine generator connection transformer 210, 211 and212, respectively, where the transformer presents an impedance Z1, Z2and Z3 to the collector system. The wind turbine generator collectiontransformers 210, 211 and 212 may be located at varying physicaldistances 215, 216 and 217 from the collection bus 205 presentingdifferent line resistance and reactance to the system (Z4, Z5 and Z6). Acommon path for one or more wind turbine generator loads may also bepresented to the collector system such as 218 (Z7) between thecollection bus 205 and wind farm main transformer 224. While theimpedances are shown for illustrative purposes as discrete elements, itis recognized that they may represent distributed line elements,representing varying distances of line.

The collector bus 205 is tied through a point of common connection to atransmission grid 225 through wind farm transformer 224. Sensing devicesat the POCC 220 may provide measured voltage, current, power factor,real power and reactive power signals to a windfarm control system. Acontrol system is provided for the windfarm. A reference command isprovided to the windfarm control system for control of real and reactivepower. However, only the reactive load reference command signal Q_(REF)230 and reactive measured load signal Q_(M) (measured) 235 are providedto summer 240. The output from summer 240 is provided to controlfunctions H(s) 250 for determining relative load distribution to theindividual wind turbine generators. Control functions H(s) 250incorporates a loss minimization algorithm whose technical effect is tominimize windfarm collector system loss by assignment of reactive loadsQ1 251, Q2 252 and Q3 253 based on losses resulting from Z1, Z2 and Z3wind turbine generator connection transformer losses, from Z4, Z5 and Z6line losses and Z7 line losses. Further the windfarm control algorithmmay be subject to various constraints, one of which may be a powerfactor of approximately 0.95 at the POCC. Such methods, however, havenot addressed loss optimization for multiple windfarms including atransmission line between the windfarms and a point of interconnection(POI) with a grid.

Accordingly, there is a need to provide voltage controls at a point ofregulation for multiple windfarms, which also provides for constraintson voltage or other system parameters at other locations on the systemof windfarms. Further there is a need optimize losses in more complexwindfarm systems, including systems of multiple windfarms feedingtransmission lines.

BRIEF DESCRIPTION OF THE INVENTION

The present invention relates to a system and method for voltage controlof a system of multiple windfarms including transmission lines.

Briefly in accordance with one aspect of the present invention a systemof windfarms is provided, adapted for controlling voltage at a referencepoint on the system of windfarms including a transmission line providinga connection to a grid for one or more coupled local windfarms whereinvoltage control at the reference point is subject to constraints onmaintaining designated system parameters within specified tolerance. Theembodiment of the system may include a plurality of local windfarms.Each of the local windfarms may include one or more individual windturbine generators WTG with an individual output transformer. Acollector system for each local windfarm including a collectortransformer joined through a network of conductors with the individualWTGs and their individual output transformers may be provided. One ormore collector systems may be joined through a network of conductors ata collector substation. The collector system transformer steps upvoltage to a level for suitable for the transmission line. One or moresubstations may be connected to one or more transmission lines furtherconnecting the collector systems to a grid. The system may furtherinclude a local windfarm controller for each local windfarm, adapted forcontrolling generation of reactive power for each individual generatorof a local windfarm to operate within the voltage and thermal limits ofthe individual wind turbine generator. The system may further include avoltage reference point on the system for voltage control and at leastone constraint point on the system in addition to the voltage referencepoint, wherein an operating parameter for the system of windfarms isconstrained by the control system. A master controller for the windfarmsystem control system is provided. The master controller is adapted forgenerating real power and reactive power command for each local windfarm controller for controlling voltage at the voltage reference pointto a designated value subject to at least one additional constraint onan operating parameter for the system of windfarms at the constraintpoint.

According to a further aspect of the present invention, a method isprovided for controlling voltage at a point of regulation on a system ofwindfarms, including a plurality of local windfarms with each windfarmwith a plurality of individual wind turbine generators (WTG), acollector system for each local windfarm including a collectortransformer and network of conductors joining individual WTGs at a pointof common coupling, and at least one transmission line providing aconnection to a grid for the plurality of local windfarms, whereinvoltage control at the point of regulation is subject to constraints onmaintaining designated system parameters within specified tolerance. Themethod includes generating, by a master controller for the system ofwindfarms, a reactive power command for a controller for each of thelocal wind farm, adapted to control voltage at the voltage regulationpoint to a designated value subject to at least one additionalconstraint on an operating parameter for the system of windfarms.

A further aspect of the present invention provides a method foroptimizing losses in an overall system that involves multiple wind farmsplus a transmission grid between the windfarms and a point ofinterconnection (POI).

BRIEF DESCRIPTION OF THE DRAWING

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates a voltage profile at various point on a 345 kvtransmission line between a point of common coupling (POCC) with awindfarm and a point of interconnection (POI) with a grid;

FIG. 2 illustrates a prior art system for minimizing losses within asingle windfarm by distribution of reactive power commands to individualwind turbines;

FIG. 3 schematically illustrates a windfarm system control schemeadapted to measuring power-related system parameters at a point ofcommon coupling for a plurality of tightly-coupled local windfarms andusing the measured parameters to control local windfarm controllers forestablishing power-related parameters at the point of common coupling;

FIG. 4 illustrates input and output parameters that may be employed bythe windfarm system control device (WCD) for controlling local windfarmoperation.

FIG. 5 illustrates one embodiment of a voltage regulator for thewindfarm system control device;

FIG. 6 illustrates a distribution function whereby total reactive powercommand developed by the voltage regulator may be assigned to theindividual local windfarms;

FIG. 7 illustrates a flow chart of a method for voltage control of amultiple windfarm system including transmission line through reactiveload distribution with constraints on additional system parameters;

FIG. 8A illustrates a control scheme for optimizing system losses for asystem of multiple windfarms including at least one transmission line;

FIG. 8B illustrates a method for shifting of a regulating point from anoriginal bus to a different bus when a parameter for the different busis limiting;

FIG. 9 illustrates a flow chart of a preferred embodiment forpredictor-corrector approach to assigning reactive commands to theindividual windfarms;

FIG. 10 illustrates a flow chart for establishing a voltage reference ata point of common coupling of a system of multiple windfarms with one ormore transmission lines for the purpose of minimizing system losses;

FIG. 11 illustrates windfarm loss characteristics for individualwindfarms versus reactive output;

FIG. 12 illustrates a loss curve for combined losses of both windfarmsusing a loss optimization function and a loss curve for combined lossfor both windfarms without the loss optimization function; and

FIG. 13 illustrates the loss-optimizing corrector output to achieve theloss minimization represented in FIG. 12.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments of the present invention have many advantages,including

The present invention may regulate the output of a system of multiplecoupled windfarms, connected to a grid of an electrical power system, soas to jointly regulate a single common point of electrical couplingthrough coordinated real power, reactive power and voltage response. Awindfarm system control device may monitor a common measurement pointfor power-related parameters (such as currents, voltage, real power,reactive power and power factor) where the parameter value at themeasurement point is an aggregate sum of the contributions for eachlocal windfarm. Line drop compensation may be applied, if necessary, tocompensate for real power losses, reactive power losses, and voltagedrops that may be required if the measurement point is not at the pointin the system at which the combined output of the windfarms is to beregulated. The windfarm system control device may incorporate a reactivepower output command that can be used to regulate voltage at the pointof common coupling. Reactive power commands to each local windfarm maybe controlled such that operating parameters on other points of thesystem are not violated. For example, while regulating voltage at onedesignated point on the system of windfarms, such as a point of commonconnection, constraints are observed for at least one other point on thesystem, such as a point on the transmission line or a collector bus fora windfarm system. A method for implementing the coordinated control isprovided.

FIG. 3 schematically illustrates a windfarm system control deviceadapted to measuring power-related system parameters at a point ofcommon coupling for a plurality of tightly-coupled local windfarms andusing the measured parameters to control local windfarm controllers forestablishing power-related parameters at the point of common coupling. Afirst local windfarm 10, a second local windfarm 15, a third localwindfarm 20, may represent a plurality of any number of local windfarmsconnected at their outputs to a point of common coupling 25 throughtransmission lines, presenting impedances Z1, Z2 and Z3 respectively.The local windfarms 10, 15, 20 each are shown within one wind turbinegenerator 35, but local windfarms may include a hundred or more windturbine generators. Each local windfarm 10, 15, 20 includes a localwindfarm controller 60. The local windfarm controller 60 may monitorpower-related parameters 63 at the output from the individual localwindfarm 65, monitor the operating status 62 of individual wind turbinegenerators and provide control signals 61 to the individual wind turbinegenerators 35 within the respective local windfarm 10, 15, 20. One ormore transmission lines 31, may connect the POCC 25 with the POI 27 withgrid 30. The transmission lines 31 may include one or more reactivecontrol devices 22 capable of adjusting reactive load on the line.

The system of windfarms may also include at least one substation,wherein an output from at least one of the individual windfarms isconnected to the substation and an output from at least one differentindividual windfarm is connected to a different substation. The outputsfrom one or more of the substations may feed one or more transmissionlines.

The grid 30 may typically present an impedance at the point of commoncoupling to the interconnected windfarms of Z_(GRID), where Z_(GRID) islarge in comparison to the impedances Z1, Z2 and Z3 presented by thelocal windfarms. Consequently, due to the tight coupling of the localwindfarms, any individual local windfarm controller trying to respond toa signal from a centralized system controller to provide an output atthe point of common coupling 25 would be competing with the other localwindfarm controllers and their control signals to implement an output atthe point of common coupling.

A plurality of sensing devices 70, 71 at the point of common coupling 25may sense a plurality of power-related parameters 85, 90, 91 at a pointof common coupling 25 (in this case also at the point of measurement26). The power-related parameters may include real power, reactivepower, voltage, line current, and power factor. The power-relatedparameters may be transmitted 95, 96 to the windfarm system controldevice 75 by various means known in the art. The windfarm system controldevice 75 may use the above-described power-related parameter values,along with other local windfarm power-related parameter signals 76 forcontrolling the output of the local windfarms 10, 15, 20 based onreference command signals 6 from the centralized system controller 5.

The windfarm system control device 75 may use the power-relatedparameters to coordinate the individual local windfarms production ofvars to regulate system quantities at the point of common connection 25.The plurality of windfarms may be controlled such that each individualwind farm 10, 15, 20 maintains its own voltage, power and VAR limits inaddition to minimizing and eliminating var and voltage oscillationsbetween these closely coupled windfarms. Further, if a point ofregulation by the centralized system controller 5 of the electric powersystem for the interconnected local windfarms is chosen to be at alocation other than the point of measurement 26 for the local windfarms,the windfarm system control device 75 for the local windfarms 10, 15,20, 21 may provide compensation for voltage drop and power loss betweenthe point of regulation and the point of measurement 26, utilizing themeasured power-related parameters and other line parameters.

FIG. 4 illustrates input and output parameters that may be employed bythe windfarm system control device (WCD) for controlling local windfarmoperation. The WCD 75 may receive a plurality of control inputs from thecentralized system controller 5. The inputs may include, but are notrestricted to, reference values for voltage (V_(ref)) 105 for a point ofregulation and specifying point of regulation 106.

Total real power 110, total reactive power 115, line voltage 120, linecurrent 125 and power factor 130 may be measured at the point of commoncoupling. Further measured parameters may be provided from theindividual windfarms, including real power (Pwf₁ . . . Pwf_(n)) 135,reactive power (Qwf₁ . . . Qwf_(n)) 140, output voltage (Vwf₁ . . .Vwf_(n)) 145, output current (Iwf₁ . . . Iwf_(n)) 150. Furthercalculated parameters such as maximum reactive power (MAXQwf₁ . . .MAXQwf_(n)) 155 and possible maximum real power POSPwf₁ . . . POSPwf_(n)160 may be provided to the windfarm WCD from the individual localwindfarm controllers. Here, the maximum reactive power 155 for anindividual local windfarm may represent the summation of the maximumreactive power capability of individual wind turbine generators and thenumber of operating wind turbine generators within the local windfarm.

Further, parameters from the transmission line such as line voltage (Vt)108 and line current (It) 109 may be measured at instrumented points onthe transmission line or may be calculated from measured quantities atpoints in the system such as the point of interconnection or point ofregulation.

Outputs from the windfarm system control device 75 may reactive powercommands (Q_(1CMD) . . . Q_(nCMD)) 175 to individual local windfarmcontrollers. The commands are established according to algorithms, whosetechnical effect is to provide voltage regulation at the point of commoncoupling.

FIG. 5 illustrates one embodiment of a voltage regulator for thewindfarm system control device 75 according to the present invention. Avoltage reference V_(REF) 105 is provided to a voltage regulator 100Aand a windfarm reactive power command 298 is provided at the output. Inone aspect, the wind turbine control system is adapted for regulatingvoltage at a point of common coupling 25 at output of a plurality oftightly-coupled local windfarms 10, 15, 20 (FIG. 3). The input voltagereference 205 may be compared at summer 225 with a voltage droop signal230 and with a line drop compensation signal V_(linedrop) 235.

The combined signal is tested by limiter 240 to maintain voltage at thelocal connection point within limits. The combined signal then iscompared in summer 245 against V_(means) 120 to generate voltage errorsignal V_(err) 250 to be applied to proportional-integral-derivative(PID) controller 255 to generate a total reactive power commandQ_(TOTCMD) 265; Q_(TOTCMD) 265 is bounded by Q limiter 260 whereQ_(LIMITS)=Σ Q_(LIMI) . . . Q_(LIMn) for the individual windfarms.Q_(TOTCMD) 265 represents the total reactive power being commanded forthe plurality of windfarms. Sending reactive power commands to the localwindfarms eliminates conflicts between the local windfarm voltageregulators. The total reactive power command Q_(TOTCMD) 265 may then beapportioned as Q_(nCMD) to the individual local windfarms according to adistribution function, to be further described.

The reference signal Vref 105 for a nominal system voltage may bedesignated by a centralized system operator. Alternatively the referencesignal Vref 105 for the nominal system voltage may be specified by anoperator for the system of windfarms. A control signal 108 (FIG. 3) fromthe centralized system operator or from the operator for the system ofwindfarms may also specify points on the system of windfarms includingthe transmission lines, which may be designated as the point of voltageregulation. The point of voltage regulation may be at the point ofcommon coupling 25, a point of interconnection 27 with the grid 30 or atother designated points on the system. The point of measurement 26 forrelevant system parameters may be at the point of common coupling 25. Ifthe point of measurement for relevant system parameters is not the pointof regulation, then the voltage drop between the point of regulation andthe point of measurement may be calculated, taking into account linelosses in voltage and power.

FIG. 6 illustrates a distribution function whereby the Q_(TOTCMD)developed by the voltage regulator for the total reactive power at thepoint of common coupling may be assigned to the individual localwindfarms. From Q_(TOTCMD) 265, a windfarm reactive power commandQ_(ICMD) . . . Q_(nCMD) 298 may be assigned by a distribution algorithm295 for each local windfarm controller. One embodiment of thedistribution algorithm 295, may utilize local maximum online reactivepower ratings provided from the individual local windfarm or theindividual local windfarm controllers to the windfarm system controldevice 75. The local windfarm or local windfarm controller may generateits local maximum online reactive power rating Q_(IONLINE-RATING) . . .Q_(iONLINE-RATING) 155 (FIG. 6), based on the number of wind turbinegenerators operating in the local windfarm and the reactive power ratingof the individual wind turbine generators. The reactive power commandprovided to windfarm i, may be described in Equation 1:

$\begin{matrix}{Q_{iCMD} = {\frac{Q_{TOTCMD} - Q_{iONLINERATING}}{\sum\limits_{n = 1}^{N}Q_{i\;{MAX}}}.}} & {{{Equation}\mspace{14mu} 1}\;}\end{matrix}$

Additional constraints may be placed on the system such that thereference voltage may be maintained at the point of regulation subjectto the maintenance of conditions at other locations on the system ofwindfarms. For example, the reference voltage at the point of regulationmay be maintained subject to maintaining voltage along the transmissionline within normal range of 0.95 PU to 1.05 PU. Or for example, thevoltage along a section of the transmission line, built to withstand avoltage of 1.10 PU may be allowed to operate within an expanded voltageband of 0.95 PU to 1.10 PU. Information related to the voltage on thetransmission line may be obtained from direct measurement on thetransmission line or calculated according to voltage drop between pointsof measurement and the point of control. Such calculations may beperformed by real-time system models.

Other constraints may be incorporated on collector buses, for example.Limits on collector bus output voltage may be required to be maintainedwithin predesignated limits. Voltage limits may be required to bemaintained within 0.95 PU to 1.05 PU.

If voltage at a point of constraint reaches a limit, then action may betaken to return voltage at the point of constraint to within theallowable limits. For example, if a collector bus exceeds apredesignated voltage limit, then reactive load may be shifted toanother collector bus operating within acceptable voltage limits andwith capability to accept additional reactive load. If no furtherreactive load can be shifted to restore collector bus voltage, thenvoltage at the point of regulation may be regulated to restore thecollector bus to within the predesignated voltage limits.

Similarly, if voltage on one or more transmission lines exceedsallowable limits, then action should be taken to restore voltage towithin limits. For multiple transmission lines such action may includeoperating reactive control devices on the transmission line whose effectis to shift load.

According to a further aspect of the present invention, a method isprovided for coordinating control of closely-coupled local windfarmsconnected at a point of common connection with an electric power grid.The method may include receiving power-related reference signals (P, Q,V, I) from a centralized system controller for the electric power gridand also receiving power-related operational signals from each of aplurality of local windfarms. The method may also include sensing aplurality of power-related parameters at a point of common connectionwith grid. According to the reference signals provided from thecentralized system controller and the power-related operational signalssupplied by the plurality of local windfarms, a plurality ofpower-related commands are generated for each of the plurality of localwindfarms. The plurality of power-related commands are transmitted toeach of the plurality of local windfarms for controlling the output ofthe individual local windfarms to produce a combined output at the pointof common coupling, or alternatively at a different point of regulation,according to the power-related reference signals.

FIG. 7 illustrates a flow chart of a method for voltage control of amultiple windfarm system including transmission line through reactiveload distribution with constraints on additional system parameters. Instep 710, local windfarms are operated within local limits according tothe local windfarm controller. In step 715, the master control device(MCD) receives power-related parameters from local windfarm controllers.In step 720, the MCD receives power-related parameters from a systemmeasurement point. Reference voltage Vref is received from a centralizedsystem control or from a local control in step 725. Vref is comparedwith Vmeas in step 730. In step 735 a total reactive power command isgenerated. In step 740, the FIG. 8B illustrates a method for shifting ofa regulating point from an original bus to a different bus when aparameter for the different bus is limiting total reactive power commandis split to commands for the individual windfarms per a distributionalgorithm. In step 745, it is determined whether a constraint isviolated at a designated point on the system (for example a collectorvoltage). If no constraint is violated, operation may continue with thereactive load split to windfarms as determined by the distributionalgorithm. If a constraint, such as collector voltage, is violated thenit must be determined in step 750 whether other windfarms includeadditional reactive load capability and are also within voltage limits.In step 755, if reactive load can be shifted to another windfarm whilemaintaining voltage in specification, then operation with the newreactive load split is continued in step 765. If no shift of reactiveload can be accomplished, then the reference voltage for the point ofregulation may be reduced in step 760 to effect voltage regulation onthe collector bus.

FIG. 8B illustrates a method for shifting of a regulating point from anoriginal bus to a different bus when a parameter for the different busis limiting. V0Ref 847 is the command to the POCC regulator 845 andV0Refo 865 is the desired value of V0Ref 847 from other inputs, e.g.loss optimization. Should this be within the limit V0Limit 885, then itis used directly; otherwise V0Ref 847 is held to V0Limit 885.

V0Limit 885 is computed based on the equipment constraints at the POCC,indicated by the parameter V0Max 886, and the status of voltage at bus 1805 (bus with constraint on its voltage such as collector bus 1 ofwindfarm 1 in FIG. 8A). V1Fbk 887 represents the measured feedback valueof the voltage at bus 1 805 and V1Max 888 is a parameter set to theequipment constraint at that bus. The Point 1 voltage regulator 891 isan integrating type of regulator where the upper limit is zero.Normally, V1Fbk 887 is smaller than V1Max 888 so the output of theregulator, dV0V1Max 889, is zero due to this upper limit. Should V1Fbk887 exceed V1Max 888, its output will become negative and V0Limit 885will be decreased below the equipment capability of the POCC 825 (FIG.8A). In steady-state, dV0V1Max 889 will settle at a value where thepoint 1 voltage is at its maximum value. When grid conditions change ina direction to relieve this constraint, then dV0V1Max 889 will becomeless negative and eventually equal zero. At that point the regulated buswill shift back to the POCC.

With multiple points of possible constraint, there would be multiplecopies of these point regulators, each summing with V0Max to defineV0Limit. The most constraining point will have regulation in thismanner.

A further aspect of the present invention provides a method foroptimizing losses in an overall system that involves multiple wind farmsplus a transmission grid between the windfarms and a point ofinterconnection (POI). In this context, the POI is remote from the pointof common coupling (POCC) of the individual local windfarms and lossesin the transmission network between the POCC and POI are considered inthe evaluation of minimizing system losses.

FIG. 8 illustrates a control scheme for optimizing system losses for asystem of multiple windfarms including at least one transmission line.Each windfarm 810, 815, 820 connects into the grid at the POCC 825,where voltage is regulated by an overall system controller 840. Thepresent example includes 3 windfarms; however, the method is applicableto any number of windfarms.

The setpoint for the voltage must be selected to minimize overall lossesin the total network from the wind turbine generators (WTG) through thetransmission line to the POI 827. An additional objective is todistribute the reactive commands among the windfarms such that the totallosses of all of the windfarms 810, 815, 820 are minimized.

The POCC regulator 845 is a standard structure, where a reactive commandto the grid (Q0Ref) 880 is determined in a manner that causes themeasured voltage V0 846 at the POCC 825 to equal a reference voltageV0Ref 847. The POCC regulator 845 would typically be an integrating typeof regulator with a closed-loop response time on the order of severalseconds.

The function of the loss optimization function is to determine both thesetpoint for V0Ref voltage 847 and to distribute the reactive commandsamong the windfarms 810, 815, 820 in a manner that minimizes losses fromthe WTGs to the POI.

Each windfarm 810, 815, 820 has a loss-optimization function asdescribed in the prior art. This distributes reactive commands to eachturbine within the local windfarm such that the total reactive output atthe windfarm terminals equals the command (e.g. Q1 850=Q1Ref 854, Q2851=Q2Ref 855, Q3 852=Q3Ref 856) and does so with minimum losses withinthat local windfarm.

A further requirement of the windfarm control is to calculate thepartial derivative (dLi/dQi) 857, 858, 859 of losses (L1, L2 and L3within that local windfarm) to the reactive command (Q1, Q2, Q3 ) and tocalculate the partial derivative (dLi/dVi) 860, 861, 862 of losses tothe POCC voltage V0 846. This is a straightforward extension of therelationships used to perform the optimization calculations within thelocal windfarm, relying on the same network data and equations. Thesepartial derivatives are sent to the overall loss minimizing control.

The losses in the transmission network are calculated based upon theprevailing power flow (P0 870 ,Q0 875) and voltage (V0) 846 measured atthe POCC 825, and known electrical characteristics of the transmissionnetwork 898 and the receiving network beyond the POI 827. Given thisinformation, calculation is made for the partial derivative oftransmission network losses with respect to voltage V0 (dL0/dV0) at theprevailing operating point. Similarly, the derivative of voltage withrespect to reactive command can be determined from the network(dV0/dQ0).

For a given total reactive output, the total loss of the windfarms isminimized when the reactive distribution is such that the partialderivatives with respect to reactive output are equal. For thethree-windfarm example, this is expressed by Equation 2:dL1/dQ1=dL2/dQ2=dL3/dQ3  Equation 2.

This fact is appreciated by considering a case where reactive command isincreased on one windfarm. To maintain the total reactive commandconstant, the sum of the reactive commands to the other two windfarmsmust be decreased by an equal amount. If Equation 2 is satisfied, thenthere is no change in total losses by this shifting of reactive command.If Equation 2 is not satisfied, then there will be a change in totallosses and the shifting should continue in a direction that reducestotal windfarm losses.

Thus the objective for windfarm distribution is to assign reactivecommands to the individual wind farms to meet the following criteria: 1.the sum of reactive commands equals the total reactive command; 2.equation 2 is satisfied in steady-state; and 3. a rapid response isprovided to reactive command Q0Ref, to enable good performance of thePOCC voltage regulator.

A preferred embodiment includes a predictor-corrector approach with thefollowing steps, performed for each windfarm, as illustrated in FIG. 9.In step 910, a predictor component is determined as fraction of totalcommand, where the fraction is determined by the relative reactivecapability of the individual windfarm relative to the total of allwindfarms. In step 920, determine a first corrector error proportionalto the sum of all corrector outputs. The gain on this error is selectedfor rapid constraint of deviation from total reactive command, typicallywith settling time on the order of a few computation cycles of thecorrector integration. In step 930, determine a second corrector errorproportional to the deviation of the individual windfarm dL/dQ term fromthe average dL/dQ of all windfarms. The gain on this error is typicallyselected for a closed-loop response on the order of 10 to 30 seconds.The sum of first and second corrector error is integrated to determinethe corrector output in step 940. Predictor and corrector components areadded in step 950 to determine a pre-limited reactive command. In step960, limits are imposed on final reactive command to respect capabilityof the equipment.

The objective in selecting V0Ref 847 is to minimize overall losses tothe POI. This minimum occurs when an incremental change in V0Ref 847will cause offsetting changes in loss between the windfarm total and thetransmission network. This fact is appreciated by considering anexample, where V0 846 is increased by 1%. If the transmission lossesdecrease by 100 kW and the windfarm losses increase by 100 kW, thenthere is no reason to change. However, if the transmission lossesdecrease by 200 kW then there is a net benefit in total losses andtherefore motivation to increase the voltage.

It is therefore neccessary to determine the sensitivity of total lossesto POCC voltage. The transmission portion is given directly by the termdL0/dV0 derived from the transmission system model. The windfarm portionrequires consideration of the consequential effect of changing the totalreactive command to achieve the POCC voltage change. This latter effectis conveyed in the term dV0/dQ0 derived from the transmission systemmodel.

The contribution of each windfarm to total loss sensitivity is given bythe Equation 3:dLWFi/dV0=dLi/dV0+FQWFi*(dLi/dQi)/(dV0/dQ0)   Equation 3,where dLWFi/dV0 represents a sensitivity of total windfarm loss forwindfarm “i” due to changing V0; and FQWFi represents a fraction ofreactive capability of windfarm “i” relative to total reactivecapability of all windfarms.

The sensitivity of total system loss to POCC voltage is then given byEquation 4: dLtotal/dV0=dL0/dV0+sum(dLWFi/dV0) for all windfarmsEquation 4.

FIG. 10 illustrates a flow chart for establishing a voltage reference(V0Ref) at a point of common coupling of a system of multiple windfarmswith one or more transmission lines for the purpose of minimizing systemlosses up to the point of interconnection. Step 1010 determinesparameters of transmission model. Step 1020 determines dL0/dV0 anddV0/dQ0 from transmission system model. Step 1030 determine dLtotal/dV0according to: dLtotal/dV0=dL0/dV0+sum (dLWFi/dV0) for all windfarms. Instep 1040 Increment V0Ref is incremented in a direction determined bythe sign of dLtotal/dV0 such that losses are reduced. Further, in step1050, V0Ref is limited with respect to equipment capability.

The rate of incrementing V0Ref 847 should be slow relative to the POCCvoltage regulator closed-loop response. Typically ramping V0Ref 847 onthe order of 1% per minute should give satisfactory results. Additionalrefinements, such as incorporation of deadband and hysteresis, would bewithin the capability of those with ordinary skill in the art.

An example of loss minimization is provided for a simulation of awindfarm system with two windfarms, wherein windfarm 1 has a reactivecontrol range of +/−10 MVAR and windfarm 2 has a reactive control rangeof +/−20 MVAR. However, the optimization scheme is not limited to anyspecific number of windfarms.

Windfarm loss characteristics for individual windfarms versus reactiveoutput are shown in FIG. 11, with a loss curve 1 1110 for windfarm 1 andloss curve 2 1120 for windfarm 2. FIG. 12 illustrates a loss curve 1 210for combined losses for both windwarms using a loss optimizationfunction as described above and a loss curve 1220 for combined loss forboth windfarms without the loss optimization function (where thereactive commands are distributed solely based on relative ratingwithout optimization). There is considerable benefit on the overexcitedend (total MVAr Q0>0) of the range for this example, where loss curve1210 is less than loss curve 1220.

FIG. 13 shows the loss-optimizing corrector output 1310 to achieve theloss minimization shown in loss curve 1210 of FIG. 12. Observe that theloss-optimizing output is limited to zero at the extreme ends of thecurve to respect the reactive limits of the two windfarms.

While various embodiments are described herein, it will be appreciatedfrom the specification that various combinations of elements, variationsor improvements therein may be made, and are within the scope of theinvention.

1. A system for controlling voltage at a voltage regulation point on asystem of windfarms comprising: a plurality of local windfarms; acollector bus for each local windfarm; at least one transmission lineconnecting a point of common coupling for the collector buses to a pointof interconnection with a grid; a local windfarm controller for eachlocal windfarm, adapted for control of a generation of real power andreactive power for each individual generator of a local windfarm; acontrol system configured to generate reactive power commands for eachlocal windfarm controller for controlling voltage at the voltageregulation point subject to at least one additional constraint on anoperating parameter of at least one of the collector bus for each localwindfarm and the at least one transmission line, wherein the controlsystem is further configured to implement a constraint algorithm todetermine when the operating parameter has exceeded a predesignatedlimit and to control the reactive power commands generated for eachlocal wind farm controller so that the at least one additionalconstraint is not violated.
 2. The system of claim 1, wherein thecontrol system is configured to receive a voltage reference setting forthe voltage regulation point from a centralized system operator.
 3. Thesystem of claim 1, wherein the voltage regulation point is establishedon a transmission line from the plurality of local windfarms to the gridat one of an intermediate point on the transmission line and a windfarmside of the transmission line.
 4. The system of claim 1, wherein thevoltage regulation point is established at the point of common coupling.5. The system of claim 1, further comprising at least one sensing deviceconfigured to measure at least one power-related parameter at a point ofmeasurement, the control system being configured to compensate for linedrops between the point of measurement and the voltage regulation pointusing the at least one power-related parameter.
 6. The system of claim1, wherein the reactive power commands for each local windfarmcontroller are apportioned from a total reactive power command, thecontrol system being configured to implement a distribution algorithm inorder to apportion the total reactive power command according to areactive power capability for each local windfarm controller.
 7. Thesystem of claim 1, wherein the constraint algorithm comprises: ameasurement of a voltage on the collector bus for each local windfarm; adetermination of when the voltage on any collector bus exceeds apredesignated value; a determination of a reactive load capability foreach local windfarm; an identification of the collector bus with voltagewithin the predesignated value and with available reactive loadcapability; and a transfer of reactive power to the collector bus withvoltage within the predesignated value and with available reactive powerload capability.
 8. The system of claim 7, wherein the constraintalgorithm further comprises a shifting of the voltage regulation pointto the collector bus with voltage exceeding the predesignated value whenthe control system is unable to restore voltage on the collector bus towithin the predesignated value by transferring reactive power to thecollector bus with voltage within the predesignated value and withavailable reactive power load capability.
 9. The system of claim 1,wherein the at least one transmission line comprises a plurality oftransmission lines and wherein the constraint algorithm includes: adetermination when a voltage on any transmission line of the theplurality transmission lines exceeds a predesignated value; adetermination of the transmission line of the plurality of transmissionlines that includes reactive load capability and voltage within thepredesignated value; and a transfer of reactive power to thetransmission line with available reactive power load capability andvoltage within the predesignated value.
 10. The system of claim 9,wherein the constraint algorithm further comprises a shifting of thevoltage regulation point to the transmission line with voltage exceedingthe predesignated value when the control system is unable to restorevoltage on the transmission line within the predesignated value bytransferring reactive power to the transmission line with availablereactive power load capability and voltage within the predesignatedvalue.
 11. The system of claim 1,: wherein the control system is furtherconfigured to implement an optimizing algorithm to assign reactive powerto each local windfarm controller to optimize system efficiency withinconstraints of local windfarm limits.
 12. A method for controllingvoltage at a voltage regulation point on a system of windfarms,comprising: generating, by a control system for the system of windfarms,a reactive power command for a controller of each local windfarm withinthe system of windfarms to control voltage at the voltage regulationpoint subject to at least one additional constraint on an operatingparameter of at least one of a collector bus for each local windfarm andat least one transmission line connecting a point of common coupling forthe collector buses to a point of interconnection with a grid; and,controlling the reactive power command generated for the controller ofeach local windfarm so that the at least one additional constraint isnot violated.
 13. The method of claim 12, comprising: setting a voltagereference for the voltage regulation point.
 14. The method of claim 12,further comprising: establishing the voltage regulation point at one ofa point on the at least one transmission line, a point of commonconnection for the system of windfarms and at least one of the collectorbuses.
 15. The method of claim 12, further comprising: measuring atleast one power-related parameter at a point of measurement; andcompensating for line drop between the point of measurement and thevoltage reference point using the at least one power-related parameter.16. The method of claim 12, further comprising: calculating a totalreactive power command for the system of windfarms; and assigning thereactive power command to each local windfarm according to a reactivepower capability for the controller of each local windfarm.
 17. Themethod of claim 12, wherein controlling the reactive power commandgenerated for the controller of each local windfarm so that the at leastone additional constraint is not violated comprises: determining whenthe operating parameter has exceeded a predesignated limit; andrestoring the operating parameter within the predesignated limit bycontrolling the reactive power command generated for the controller ofeach local windfarm.
 18. The method of claim 12, wherein controlling thereactive power command generated for the controller of each localwindfarm so that the at least one additional constraint is not violatedcomprises: measuring a voltage on the collector bus for each localwindfarm; determining when the voltage on any collector bus exceeds apredesignated value; determining a reactive load capability for eachlocal windfarm; identifying the collector bus with voltage within thepredesignated value and with available reactive load capability; andtransferring reactive power to the collector bus with voltage within thepredesignated value and with available reactive power load capability.19. The method of claim 18, further comprising shifting the voltageregulation point to the collector bus with voltage that exceeds thepredesignated value when the control system is unable to otherwiserestore voltage on the collector bus to within the predesignated valueby transferring reactive power to the collector bus with voltage withinthe predesignated value and with available reactive power loadcapability.
 20. The method of claim 12, wherein the at least onetransmission line comprises a plurality of transmission lines andwherein controlling the reactive power command generated for thecontroller of each local windfarm so that the at least one additionalconstraint is not violated comprises: determining when a voltage on anytransmission line of the plurality of transmission lines exceeds apredesignated value; determining the transmission line of the pluralityof transmission lines that includes available reactive load capabilityand voltage within the predesignated value; and transferring reactivepower to the transmission line having available reactive load capabilityand voltage within the predesignated value.
 21. The method of claim 20,further comprising shifting the voltage regulation point to thetransmission line with voltage exceeding the predesignated value whenthe control system is unable to restore voltage on the transmission linewithin the predesignated value by transferring reactive power to thetransmission line with available reactive power load capability andvoltage within the predesignated value.
 22. The method of claim 12optimizing assignment of reactive power to the controller for each localwindfarm to allow for system efficiency within constraints of localwindfarm limits.